had some unknowing people mess it up by attacking first out of party so i couldnt loot.. waited a while went back to refuge after reading a post. followed the horseback riders out of town (south) and sure enough, there they were.  tried to pull towards the riders, didnt work.  too far away.  so i tried it on my own as a 33 frost mage.

blizzard in front of the group, let em walk into it

frost nova and back off if they get too close

when that wears off or if u wanna cut time, arcane nova til the courier is left, then finish off the courier

... easy as pie, the guards have very little hp